1. Field of the Invention
This invention relates to a hydraulically actuated mechanism for effecting the firing of a well perforating gun, and particularly a firing mechanism which is responsive only to two successive increases in the well annulus pressure above a set packer to two predetermined values.
2. Summary of the Prior Art
The perforating of well casings and the adjoining production formation by the detonation of a plurality of explosive-shaped charges is the predominate method employed to initiate production flow from a producing formation traversed by the well casing. As is well known in the art, it is highly desirable that the well be perforated in a so-called "underbalanced" condition; i.e., the pressure in the annulus between the perforating gun and the bore of the well casing is maintained at a substantially lower value prior to firing the perforating gun than the formation pressure, thus contributing to an immediate flow of production fluid from the perforated formation. Such underbalanced condition is generally obtained by the setting of a packer above a tubing-carried perforating gun to position the perforating gun adjacent to the production formation. Thus, the casing annulus above the packer is isolated from the casing annulus below the packer. One or more axially extending fluid passages are provided in the body of the packer which communicate between the casing annulus above the packer and an inlet passage of a crossover tool located below the packer. Such cross-over tool transmits the casing annulus fluid pressure to the interior of the firing mechanism for the perforating gun, while connecting the tubing bore to the casing annulus below the packer through radial ports. With this arrangement, a pressure differential can be readily obtained between the casing annulus pressure above the packer and the tubing pressure, and such differential has been employed to effect the actuation of the firing mechanism for the perforating gun.
Those skilled in the art will further recognize that in the initial drilling of the well, it is highly desirable to test the various production formations traversed by the well. Such testing has to inherently involve the perforating of at least a portion of the production formation, and it has been common to effect such perforation by a wireline-carried perforating gun and to control the well by drilling mud. This method is highly unsatisfactory for the reason that the drilling mud effects a contamination of the newly formed well perforations and reduces the productive capability of the particular perforations. Additionally, the opportunity to perforate in an underbalanced condition and to get a true measure of the fluid pressure of the perforated production formation does not exist.
Accordingly, prior art attempts to obtain underbalanced perforation in the so-called drill stem testing operations have involved the use of a tubing-mounted perforating gun and a packer which is inserted in the newly cased well on the bottom of the drill pipe or drill stem. Included in this hookup is a ball valve which is normally closed as the drill stem, packer, cross-over tool, and perforating gun are run into the well, thus maintaining a lower pressure in the interior of the test string. Such ball valve must, of course, be opened to establish an underbalanced condition in the well annulus adjacent the perforating gun and to accommodate the flow of fluid from the formation being tested upwardly in the drill stem, at least to the point where pressure recorders and other instruments are mounted, in order to evaluate the productivity of the particular area of the well being perforated.
Such ball valves are normally opened by applying an increased pressure to the fluid existing in the well annulus above the packer or, in other words, by establishing a differential between the well annulus pressure and a hydrostatic reference pressure trapped in the ball valve. This pressure differential therefore cannot be utilized to effect the activation of a conventional fluid pressure operated firing mechanism for the perforating gun because the gun could fire prematurely before the ball valve is completely opened. It therefore becomes not only desirable, but highly necessary that a pressure-activated firing mechanism for a perforating gun employed in underbalanced drill stem testing be capable of responding only to an annulus fluid pressure other than the differential between the casing annulus pressure and the pressure within the drill pipe or drill stem; and other than the annulus pressure at which the ball valve is operated.
The prior art has also resorted to utilization of a trapped fluid pressure disposed within a tubular housing assemblage surrounding a piston-type firing hammer or firing pin. The required firing movement of the hammer or firing pin was then derived by achieving a predetermined differential between the annulus fluid pressure above the packer and the trapped fluid pressure. Unfortunately, when the fluid pressure operated test valve was opened and the pressure in the annulus surrounding the firing mechanism was reduced to achieve underbalancing, such reduction produced a substantial pressure differential across the walls of the chambers enclosing the trapped fluid and such chamber inherently expanded sufficiently to significantly reduce the fluid pressure of the trapped fluid so that premature firing was again a distinct possibility.
It is therefore highly desirable that a firing mechanism be provided which is responsive to the readily controlled annulus fluid pressure existing above the packer but which will not effect the firing of the perforating gun upon an increase in annulus fluid pressure above the packer to a level sufficient to open the pressure-operated test valve.
It is also desirable to be able to remove the packer from the firing mechanism in the event the firing mechanism or the perforating gun becomes lodged in the well. Thus, a detachable connection between the packer and the firing mechanism is another desirable attribute of such apparatus.